This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application entitled Tension Mask for Color Picture Tube and Method of Manufacturing the Same and Exposure Mask for Making the Tension Mask earlier filed in the Korean Industrial Property Office on Apr. 20, 2000, and there duly assigned Serial No. 20994/2000 by that Office.
1. Field of the Invention
The present invention relates to a color cathode-ray tube (CRT), and more particularly, to a tension mask having a color selection function, which is secured into the panel of a cathode-ray tube, a method for manufacturing the tension mask, and an exposure mask for use in the manufacture of the tension mask.
2. Description of the Background Art
In color cathode-ray tubes for television and computer displays, three electron beams emitted from an electron gun land onto a phosphor screen installed on the inner side of a panel through apertures of a mask having a color selection function, to excite red, green, and blue phosphors deposited on the phosphor screen, thereby forming images.
In a color cathode-ray tube, which forms images with the above-mentioned structure, the mask having a color selection function includes a dot mask for computer monitors, and a slot mask (or slit mask) for televisions. The dot mask and the slot mask are designed to have a predetermined curvature corresponding to a curvature of the phosphor screen, which has been curved taking into account a deflection trajectory of electron beams onto the phosphor screen.
Such masks are made of steel foil having a thickness of 0.1-0.25 mm (millimeters). A plurality of apertures are formed in the steel foil via etching, and then the steel foil is molded to have a predetermined curvature. If the curvature of the mask is less than a predetermined level, the mask is readily subjected to a permanent thermal distortion. As a result, the mask cannot perform its intrinsic color selection function. There is an increasing need for flat cathode-ray tubes. However, there are limitations in the manufacturing of flat cathode-ray tubes having the above-mentioned configuration.
A slot mask suitable for flat cathode-ray tubes, which is free from doming caused by thermal expansion, is disclosed in U.S. Pat. No. 4,942,332 issued to Adler et al. for Tied Slit Mask for Color Cathode Ray Tube. The slit-type foil tension mask includes a series of parallel strips separated by slits. The strips are loosely coupled by widely spaced ties.
Another mask, which is disclosed in U.S. Pat. No. 4,926,089 issued to Moore for Tied Slit Foil Shadow Mask with False Ties includes a plurality of tie bars interconnecting adjacent strips and to define slots, and a plurality of false bars extending between the adjacent strips to face each other, but not interconnecting the adjacent strips.
In general, such a tension mask having the above configuration is manufactured by photolithography. In particular, a photosensitive layer is deposited over both sides of a steel foil, exposed to an exposure light using an exposure mask to form a predetermined pattern, and then etched, so that a complete mask is obtained.
For the tension mask formed by the earlier techniques, the width of the slot between the adjacent strips at the electron beam emitting surface of the mask, is larger than the width of the slot at the electron beam entering side of the mask. An etching boundary, where etching from the upper and lower sides of the steel foil stops, is located close to the bottom surface of the resultant tension mask. That is, the distance from the top surface of the strip to the etching boundary is larger than the distance from the bottom surface of the strip to the etching boundary. As a result, the incident angle of electron beams passing the slot is small, so that the amount of beam passing through the slot decreases.
On the other hand, because the gap between the adjacent upper second light shielding portions of the upper exposure mask is equal to that between the adjacent lower second light shielding portions of the lower exposure mask, etching boundaries for the false bars of the tension mask, which extend from the adjacent strips, is located at a position separated by the same distance from the top and bottom of the strips. Unfortunately, the etching boundaries located in the middle of each false bar cause clogging of the gap between the adjacent false bars in the manufacture of the mask. In addition, if the gap between the false bars is widened so as to prevent clogging of the gap, a problem of false bar visibility occurs when the mask is adopted in a cathode-ray tube.
On the other hand, the gap between the false bars is limited by the thickness of the steel foil used. Earlier etching techniques applied in the manufacture of masks is insufficient to form a mask having a fine pattern, which is not shown on the screen when the mask is secured into the panel of a cathode-ray tube. In particular, although a steel foil which is thin ensures a smaller gap between adjacent false bars, the use of the thin steel foil increases the manufacturing costs, and lowers strength of the mask.
It is therefore an objective of the present invention to provide a tension mask for a color cathode-ray tube (CRT), in which variations in gaps between facing dummy bridges are reduced by adjusting the shape and thickness of real bridges and dummy bridges of the tension mask, thereby preventing the problem of bridge visibility.
It is another objective to provide a method for manufacturing a tension mask for a color cathode-ray tube, in which a desired pattern of the tension mask including slots, real bridges and dummy bridges can be obtained irrespective of the thickness of a steel foil selected to form the tension mask.
It is yet another objective to provide an exposure mask for use in manufacturing the tension mask.
Accordingly, to achieve the above objectives, there is provided a tension mask for a color cathode-ray tube including a series of parallel strips separated by a predetermined distance from each other, a plurality of real bridges intersecting adjacent strips among the series of parallel strips to define slots through which electron beams pass, and a plurality of dummy bridges located in the slots, partially extending between but not intersecting the adjacent strips, facing each other, where an etching boundary of each of the dummy bridges is located below the middle of the strips.
It is preferable that the plurality of real bridges are recessed by a predetermined depth from the top surface thereof The distance from the bottom of the strips to the etching boundaries of the dummy bridges may be 0.25 times smaller than the thickness of the strips. It is preferable that the thickness of each of the real bridges at the recessed center thereof is approximately the same as the distance from the bottom of the strips to the etching boundaries of the dummy bridges.
In another embodiment of the present invention, a tension mask for a color cathode-ray tube includes a series of parallel strips separated by a predetermined distance from each other, a plurality of real bridges intersecting adjacent strips among the series of the parallel strips to define slots through which electron beams pass, and a plurality of dummy bridges located in the slots, partially extending between but not intersecting the adjacent strips, facing each other, where an etching boundary of each of the dummy bridges is located below the middle of the strips, the tension mask including a pair of first rounded portions formed with a first thickness at the beam emitting side of each of the slots, partially extending from the adjacent strips, and a pair of second rounded portions formed with a second width at the beam entering side of each of the slots, partially extending from the adjacent strips.
A method for manufacturing a tension mask for a color cathode-ray tube (CRT) includes depositing photosensitive layers over the top and bottom surfaces of a steel foil, aligning an upper exposure mask with a pattern including a series of parallel upper light transmission portions arranged in lines over the top surface of the steel foil, aligning a lower exposure mask with a pattern over the bottom surface of the steel foil, the pattern of the lower exposure mask including a series of parallel lower light transmission portions arranged in lines, a plurality of first light shielding portions intersecting adjacent lower light transmission portions among the series of the parallel lower light transmission portions, and a plurality of second light shielding portions partially extending between the edges of the adjacent lower light transmission portions, exposing the photosensitive layers uncovered with the lower and upper exposure masks using an exposure light source, removing the upper and lower exposure masks from the steel foil and developing the photosensitive layers remaining on the steel foil, and etching the steel foil which has undergone the developing process.